Electric vehicle (EV) batteries have limited life spans, which are significantly impacted by their operating conditions, including operating temperatures, partial cycles, charging and discharging rates and profiles, etc. Over time and with use EV batteries become no longer able to meet EV performance standards, such as the ability to adequately hold a charge or supply a desired current.
The ability of any particular EV battery to meet given performance standards is sometimes called the state-of-health of that EV battery. The state-of-health of an EV battery is a function of variables related to its design, its manufacture, its history of use, the environmental conditions in which it is used and stored, the charge-discharge rates, profiles and cycles it has experienced, and other factors. Typically, the performance standards for EV batteries to be used in electric vehicles include maintaining 80 percent of total usable capacity and achieving a resting self-discharge rate of only about 5 percent over a 24-hour period. When EV batteries can no longer meet performance standards, they are typically replaced.
EV batteries can, however, have a second-life, as they are still able to perform sufficiently to serve less-demanding applications, such as stationary energy-storage services and other applications that require less-frequent battery cycling. Some EV battery second-life applications include, for example, providing reserve energy capacity to maintain a utility's power reliability at lower cost by displacing more expensive and less efficient assets (for instance, old combined-cycle gas turbines), deferring transmission and distribution investments, and taking advantage of power-arbitrage opportunities by storing renewable power for use during periods of scarcity, thus providing greater grid flexibility and firming to the grid.
Such second-life uses may encounter problems when a plurality of second-life EV batteries having different states-of-health are grouped together to provide stationary storage capacity in these second-life applications. Where the states-of-health of grouped together second-life EV batteries are substantially different, such differences can impact how they perform when connected to the grid. For example, their performance may be inefficient.
Electric vehicle battery second-life system systems and methods are therefore needed for managing the states-of-health of a plurality of EV batteries for respective second-life uses.